Impact of Natural Organic Matter Competition on the Adsorptive Removal of Acetochlor and Metolachlor from Low-Specific UV Absorbance Surface Waters

Although activatedcarbon adsorption is a very promising processfor the removal of organic compounds from surface waters, the removalperformance for nonionic pesticides could be adversely affected byco-occurring natural organic matter. Natural organic matter can competewith pesticides during the adsorption process, and the size of naturalorganic matter affects the removal of pesticides, as low-molecular-weightorganics directly compete for adsorbent sites with pesticides. Thisstudy aims to investigate the competitive impact of low-molecular-weightorganics on the adsorptive removal of acetochlor and metolachlor byfour commercial powdered activated carbons. The adsorption featuresof selected powdered activated carbons were evaluated in surface watersamples collected from the influent stream of the filtration processhaving 2.75 mg/L organic matter and 0.87 L/mg-m specific UV absorbance.The adsorption kinetics and capacities were examined by employingpseudo-first-order, pseudo-second-order, and intraparticle diffusionkinetic models and modified Freundlich and Langmuir isotherm modelsto the experimental data. The competitive removal of acetochlor andmetolachlor in the presence of natural organic matter was evaluatedfor varied powdered activated carbon dosages on the basis of UV andspecific UV absorbance values of adsorbed organic matter. The adsorptiondata were well represented by the modified Freundlich isotherm, aswell as pseudo-second-order kinetics. The maximum organic matter adsorptioncapacities of the modified Freundlich isotherm were observed to be120.6 and 127.2 mg/g by Norit SX Ultra and 99.5 and 100.6 mg/g byAC Puriss for acetochlor- and metolachlor-containing water samples,respectively. Among the four powdered activated carbons, Norit SXUltra and AC Puriss provided the highest natural organic matter removalperformances with 76 and 72% and 71 and 65% for acetochlor- and metolachlor-containingsamples, respectively. Similarly, Norit SX Ultra and AC Puriss werevery effective for adsorbing aromatic organics with higher than 80%specific UV absorbance removal efficiency. Metolachlor was almostcompletely removed by higher than 98% by Norit SX Ultra, Norit SXF Cat, and AC Puriss, even at low adsorbent dosages. However, an adsorbentdose of 100 mg/L and above should be added for all powdered activatedcarbons, except for Norit SX F Cat, for achieving an acetochlor removalperformance of higher than 98%. The competition between low-molecular-weightorganics (low-specific UV absorbance) and acetochlor and metolachlorwas more apparent at low adsorbent dosages (10-75 mg/L).